Processes for the synthesis of polyol fatty acid polyesters by the transesterification of a polyol are well known in the art. For example, the Rizzi et al. U.S. Pat. No. 3,963,699 discloses a solvent-free transesterification process comprising two main steps, each of which is conducted in a batch reactor. In the first step, a mixture of polyol, a fatty acid lower alkyl ester, an alkali metal fatty acid soap, and a basic catalyst are heated to form a homogenous melt of partially esterified polyol and unreacted starting materials. In a second step, excess fatty acid lower alkyl esters are added to the reaction product of the first step to form the polyol fatty acid polyester. Rizzi et al. further disclose that a lower alcohol is formed as by-product of the reaction and, in order to promote the reaction, the alcohol by-product is preferably removed. Many removal techniques are acknowledged by Rizzi et al. as being known in the art; Rizzi et al. indicate that vacuum removal, both with and without an inert gas sparging, has been found to promote the reaction, and that simple distillation under atmospheric pressure may also be sufficient.
The Volpenhein U.S. Pat. Nos. 4,517,360 and 4,518,772 disclose further solvent-free transesterification processes for producing higher polyol fatty acid polyesters. In U.S. Pat. No. 4,517,360, Volpenhein discloses the use of potassium carbonate, sodium carbonate or barium carbonate as a catalyst and the use of a fatty acid methyl, 2-methoxy ethyl or benzyl ester. In U.S. Pat. No. 4,518,772, Volpenhein discloses the use of preferred molar ratios of soap to polyol of from about 0.6:1 to about 1:1 in the first step of the two step process. Volpenhein also employs a batch reaction process and discloses the advantage of removing lower alcohol by-product to promote the transesterification reaction.
The Buter U.S. Pat. No. 5,043,438 discloses a process for the synthesis of polyol fatty acid esters by reacting a polyol and a fatty acid lower alkyl ester under substantially solvent-free conditions. Buter discloses that the process employs a prereactor in which the reaction mixture is in steady state with mass-balanced in-going reactant streams and out-going product streams having a polyol conversion of 1% or more, and a nonagitated column main reactor, which in the examples was a three-tray column reactor with counter-current stripping. Buter further discloses that the process reduces initial viscosity and/or de-mixing problems caused by the heterogeneous nature of the reactant mixture and by the use of soap emulsifiers. However, the process disclosed by Buter is not suitable for use on an industrial scale owing to the large quantities and high flow rate of stripping gas, i.e. nitrogen, and large reaction times which would be required therein.
Polyol fatty acid polyesters are increasingly being employed in various applications. Particularly, there has been a significant increase in the use of polyol fatty acid polyesters as low-calorie fats in many food products. Accordingly, the demand for polyol fatty acid polyesters suitable for human consumption is rapidly increasing. As a result, processes for more efficient and economical synthesis of polyol fatty acid polyesters are necessary and desirable.